The present invention relates to a method for treating used or exhausted photographic fixing solution and, more especially, to a novel and useful method of electrolysis treatment of used fixing solution by which silver ion contained in the solution is effectively removed as a precipitate of silver sulfide and simultaneously or thereafter COD value (Chemical oxygen demand) of the solution is effectively decreased.
Fixing solution, which is used in photographic processes, is gradually deteriorated by accumulating therein silver halide as soluble salts during use.
Therefore, in photographic laboratories or the like in which a large quantity of fixing solution is successively used, the used fixing solutions are usually recycled by removing silver component by means of desilvering (or silver removing) apparatus.
However, the waste water discharged from the photographic laboratories or hospitals, schools, research institutes, etc., in which a relatively small amount of the fixing solution is used, is usually containing silver halide and, moreover, the COD value of such a waste water is extremely high mainly due to sodium thiosulfate, which is one of the main components of the fixing solution, and it reaches, sometimes, 50,000-100,000 ppm. Accordingly, it has been required that the reduction of the COD value and the removal of silver are attained effectively each other.
The conventional method for treating the use fixing solution is a method in which sulfide, such as sodium sulfide, is added into the used fixing solution to precipitate and recover silver component in the solution as silver sulfide and then chlorine gas or hypochlorite solution is added thereto to decompose thiosulfate ion. As is clear from the above conventional meethod, in the known methods, silver-removing treatment and COD-lowering treatment are carried out separately, and accordingly, the known methods have various defects such as difficulty in operation and expensiveness of apparatus and running cost. Among the known methods, there is a method in which the used fixing solution is electrolyzed to recover silver component as metal silver. However, such a known electrolytic silver recovery can not attain reduction of COD value. Moreover, although the known electrolytic method is an effective silver-recovering method in case when the concentration of silver component in the solution is relatively high, it is difficult to recover metal silver of high purity in case when the concentration of silver component is less than 0.5 g/l. In this case, while metal silver is deposited, highly colloidal silver sulfide is also deposited simultaneously to cover over the surface of cathode and colloidal sulfur is also deposited to cover over the surface of anode. As a result, the bath voltage is gradually increased, the efficiency of electrolysis is gradually lowered and finally the continuation of electrolysis becomes impossible.
An object of the present invention is to overcome the problems and disadvantages of the conventional methods.
A further object of the present invention is to provide a method for carrying out effectively silver recovery and the reduction of the COD value is used fixing solution by means of quite simple apparatus and easy operation.
Other objects and advantages of the present invention will become apparent hereinafter.